A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
An illumination system is provided for receiving a beam of radiation from a radiation source and for supplying a conditioned beam of radiation, referred to as the projection beam, having a desired uniformity and intensity distribution in its cross-section for patterning by the reticle. The source and the lithographic apparatus may be separate entities, for example when the source is a plasma discharge source. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is generally passed from the source to the illumination system with the aid of a radiation collector comprising, for example, suitable collecting mirrors and/or a spectral purity filter. In other cases the source may be an integral part of the apparatus, for example when the source is a mercury lamp.
The illumination system may comprise components for adjusting the angular intensity distribution of the beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. The illumination system may also include an attenuator for varying the overall intensity of the projection beam in order to control the radiation dose at the wafer, for example to enable the dose at the wafer to be maintained constant while the source intensity drifts during multiple scans, or to change the dose at the wafer during scanning of an image while keeping the scan speed constant. However known attenuators used in such an application tend to adversely affect the operation of the rest of the illumination system, for example by changing the position of the illumination beam, and/or are difficult to control in such a manner as to accurately adjust the dose at the wafer and to control the speed of adjustment of the dose might be an issue, thereby adversely affecting the throughput and yield of wafers.
It is known to provide an attenuator in the form of a blade having three separate regions each of which has a different transmission characteristic, the attenuator being adjustable so as to move one region at a time into the radiation path, so as to allow three different discrete levels of attenuation depending on the position of the blade. However the use of only three different attenuation levels, and a fourth level in which no attenuation is provided corresponding to the blade being moved out of the radiation path, may be insufficient to maintain the required high dose accuracy at the wafer and high wafer throughput based on variation of this dose. Apart from the fact that such attenuators do not provide continuously variable levels of attenuation, they have the disadvantage that cooling may be complicated because of the complexity of the attenuator blades or poor cooling capabilities of gas cells. Furthermore the use of a blade in the beam may adversely affect certain factors determining the performance of the system, such as the uniformity of the projection beam.